#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>

int
main(int argc, char **argv)
{
	int in, parity = 0;

	while (scanf("%d", &in) == 1)
		parity ^= in;

	printf("%d\n", parity);

	return 0;
}
